The present invention relates to thick welded steel pipe of large diameter and production thereof, and more particularly to a seam-welding method therefor which is employed in producing steel pipes by UOE Process.
Production of steel pipe by the UOE Process as a method of producing thick welded steel pipe of large diameter is known. In such pipe making method, flat blankwork is progressively rounded to a U-shape and then to an O-shape having a single longitudinal seam. Multipole submerged arc welding using molten flux has been used on the inner and an outer surfaces of the seam to weld the pipe. In view of efficiency, this method is suitable to the process of the UOE steel pipe which has become thicker in cross section. However, due to the characteristics of the molten flux that viscosity and melting point become lower at high temperatures, as the cross section of the initial blankwork becomes thicker, the bead formation, of the seam deteriorates. Further, since the welding input heat becomes larger, toughness at the welded seam is deteriorated.
In these circumstances, there have been proposed certain seam-welding methods for use on thick blankwork of large diameter. One of them is as disclosed in Japanese Laid-Open application No. 130,241/78 wherein the O-shaped blankwork is welded on first layers of the inner and outer surfaces by an oscillation MIG welding method using small diameter wire with low welding current on narrow electrodes and the surface layer is finished by a submerged arc welding method. Another is as seen in Japanese Laid-Open application No. 130,242/78 wherein the material is prepared with the welding grooves having a root face of 3-8 mm at a center within 1/3 of the thickness. The grooves are symmetrical on the inner and outer sides thereof. The welding is carried out with the MIG welding up to 0.5-5.0 mm in depth of the welding grooves and performed on the final layer with a submerged arc welding method involving both the out wires and flux in the groove joint.
However, since both methods each apply, in combination, MIG welding and submerged arc welding on the inner and outer surfaces of the material, a magnetic blowing phenomenon occurs especially upon welding on the first layer and causes the bead formation thereon to be inferior. In order to avoid such occurrence, the welding facilities are restructured in a complicated manner including such changes as rearrangement of guide rollers, electrical wires and so on for providing a forward welding. Since the method of Japanese Laid-Open application No. 130,241/78 is by MIG welding with narrow wire, the efficiency is not preferable, and since the method of Japanese Laid-Open application No. 130,242 deals with submerged arc welding on the final layer as adding the steel piece, the welded metal is generated with segregation and the material is lowered in its properties.
The present invention has been devised to eliminate such problems involved in the conventional seam-welding methods for producing thick welded steel pipe of large diameter.
It is an object of the invention to provide thick welded steel pipe of large diameter having well conditioned micro-substructure and high toughness on the welded portions.
It is another object of the invention to reduce the cross sectional area of the welded seam on the inner surface in order to reduce the welding input heat and to avoid deterioration of the bead formation owing to the characteristics of the molten flux thus causing neither magnetic blowing at welding nor inconvenience thereby, so that pretty bead appearance and high toughness may be ensured.
It is a further object of the invention to carry out a padding on the edge of the larger cross sectional area at the outer surface by utilizing deep melting penetration and low input heat of the MIG welding by means of the combination of the MIG welding and the submerged arc welding, so that the outer surface of the seam is finished with a pretty bead appearance which is one of the merits of the submerged arc welding, to bring about the high toughness on the welded portions.